A solid state laser, such as a Nd:YAG laser, is typically optically pumped and may be pumped by one or more semiconductor diode lasers or laser arrays. One pumping configuration, called a "side pumped laser" configuration, is disclosed in U.S. Pat. No. 4,575,854 to Martin. The patent describes a laser with a centrally located Nd:YAG rod and a plurality of diode bar arrays arranged circumferentially around the rod. Not all arrays are driven at the same time, but instead are switched between sequential bars, so that the arrays may operate uncooled with a low duty cycle and the rod output is a continuous wave. An advantage of this configuration is that the large surface area on the side of laser rod enables use of high power laser diode arrays resulting in high power output from the laser rod. The large amount of space around the side of the rod also enables thermoelectric coolers and heat sinks to be used so as to maintain the pump light wavelength from the laser arrays close to an absorption band of the rod.
However, in a side pumped laser configuration the rod volume is unhomogeneously pumped. Pumping is very strong near the rod surface where the light is injected so the pump volume is not well confined to the deiired TEM.sub.00 cavity mode. As a result, higher order transverse modes lase and the output may exhibit spikes due to gain switching, especially at the beginning of a lasing pulse (before a steady state is attained). Attempts to maintain single TEM.sub.00 mode operation by expanding the mode volume, for example, can result in a great increase in laser threshold and lower conversion efficiency.
A second pumping configuration, called an "end pumped laser" configuration, is disclosed in U.S. Pat. No. 4,653,056 to Baer et al. That patent describes a laser with a Nd:YAG laser rod together with a laser diode array toward the rear. A collimating lens converts the diverging beam from the laser diode array into a substantially parallel beam, then a lens focuses the beam into the back end of the rod. End pumped lasers are very efficient due to the full confinement of the pumped volume to the desired TEM.sub.00 mode. However, laser diode arrays have a very high aspect ratio. The emitting region is thin, on the order of 1 .mu.m thick, in a direction perpendicular to the active region and can be extended to a width of several millimeters or more in a direction parallel to the active region to increase pump power. It is difficult to end couple the highly asymmetric light from the laser array into the desired TEM.sub.00 cavity mode of the rod. The use of coupling optics and expansion of the cavity mode volume overcomes this problem for laser array widths only up to several hundred microns. Accordingly, the end pumped laser's output power is limited.
For communication and other applications it is desirable that a laser be modulated at a high rate with well controlled pulses. Presently, solid state lasers are modulated in various ways using acousto-optics, electro-optics or other effects. To obtain short pulses, Q-switch or cavity dumping techniques are used which typically also employ an electro-optic or acousto-optic cell to control the output pulses. However, these techniques require extra elements in the laser. Low frequency pulsed modulation can also be achieved by using pulsed lamp pumping. The repetition rates of this technique are usually less than 100 Hz. Semiconductor diode lasers, and diode laser arrays can also be modulated at high rates near threshold by varying the injection current used to pump these devices. However, modulating a solid state laser by means of diode laser array optical pump sources has been generally unsuccessful due to delays between optical pumping and laser output on the order of 10 microseconds or longer which limits the obtainable modulation frequency to tens of kilohertz. An additional problem with side pumped lasers is the spiking due to gain switching, which limits their utility since optical communications and other applications require stable pulses in the TEM.sub.00 mode.
It is an object of the present invention to produce a laser with high power output, a stable transverse mode, and suppressed gain switching.
It is another object of the present invention to produce a laser which can be modulated at a high rate with no delay and little or no spiking, and which allows many types of modulation codes to be used.